The major objective of this project is to uncover the molecular mechanisms of a variety of genetic rearrangements. The transposition reaction of bacteriophage Mu is studied as a model system. Critical steps in Mu transposition are a pair of DNA cleavages and strand transfers which generate a branched DNA intermediate. Efficient formation of this intermediate requires the phage-encoded MuA and MuB proteins and the E. coli-encoded HU and IHF proteins, ATP and Mg++. The MuA protein interacts with two distinct types of DNA sequences, one type is at the ends of the Mu genome while the other lies internally at the Mu operator. Interactions involving multiple MuA molecules, accessory protein factors and sequences on the donor DNA lead to formation of a stable protein-DNA complex in which the two Mu ends are synapsed by a tetramer of MuA. Next, a pair of single strand cuts are made to expose the 3' ends of the Mu sequence. This cleaved donor DNA remains tightly associated with the MuA tetramer and this complex efficiently captures a "target" DNA molecule provided it is bound by MuB protein. A staggered cut is introduced into the target DNA and the two 5' ends are joined to the 3' ends of the Mu end sequences in a concerted reaction. Evidence has been obtained that this reaction takes place by one-step transesterification mechanism. The assembly process and the functional organization of the MuA tetramer-Mu DNA complex have been studied by making use of a variety of mutant MuA proteins with missing functional domains. The MuB protein, an ATPase, selectively stimulates utilization of intermolecular target DNA molecules which do not carry Mu end sequences. The MuB protein dissociates preferentially from DNA molecules bound by MuA protein in a process that depends on ATP hydrolysis, preventing self-destruction of Mu DNA by transposition into the Mu sequence.